Determine the peripheral stimulus threshold, the first amperage value where the M-wave exceeds 0.2 millivolts. If the M-wave exceeds the desired 0.2 millivolts target amplitude on three successive stimuli, then decrease the amperage. To determine the optimal coil trajectory for transcranial magnetic stimulation, or TMS, set an initial coil position by placing the coil on the participant's head and recording the coil trajectory.
Ensure that the center surface of the coil is tangential to the scalp. Align the midline of the coil at 45 degrees to the mid-sagittal plane of the participant's head. On the CTMS stimulator, set the pulse type selector to monophasic positive to induce a PA current in the underlying neural tissue.
Next, set the M ratio to 0.2 and the stimulus intensity to 30%of the maximal stimulator output. Finally, set the pulse width to 120 microseconds. Deliver three to five TMS stimuli while the participant maintains a slight contraction of the first dorsal interosseous, of FDI, muscle.
If no motor evoked potential, or MEP, elicits, increase the stimulator intensity by 10%and deliver three to five additional TMS stimuli. Increase the stimulator intensity until an MEP of at least 0.2 millivolts is consistently elicited to every stimulus or until the stimulator intensity reaches 60 to 70%of the maximal stimulator output. If no reliable MEP elicits, keep the stimulation parameters constant and move the TMS stimulator in a circle with an approximately two centimeter diameter around the original stimulation site.
Once a reliable MEP elicits, record the coil position. Confirm the FDI motor hotspot by keeping the stimulation parameters constant and moving the TMS stimulator two centimeters north, east, south, and west of the current coil location. Record the new coil position and trajectory if a consistently larger MEP elicits at any of the four quadrants.
Use the new coil position and trajectory as the cortical motor hotspot.
